1. Field of the Invention
The invention relates generally to radio pager systems and, more particularly, it relates to improvements in a radio pager system and a pager terminal apparatus used in this system, in which a registered caller accesses a pager base station over the public telephone line, and calls a desired pager terminal via the pager base station.
2. Description of the Related Art
A pager system is one of the systems for calling a person who is absent. A radio pager system particularly provides a service of calling a person by radio who is absent, and transmitting a message to him. One example is "pocket bell service" provided by Nippon Telegraph & Telephone Corporation (NTT) in Japan.
FIG. 1 is a typical diagram showing the configuration of a conventional pager system. Referring to FIG. 1, the radio pager system includes a public telephone line 1 used by a caller, an exchange 2, a pager base station 3 for transmitting a pager signal, and a pager terminal apparatus 4, which a callee carries about, for receiving a pager signal from the pager base station 3.
The system shown in FIG. 1 operates as follows. The registered caller accesses the pager base station 3 over the public telephone line 1. The pager base station 3 gives the caller instructions as to the procedure of calling in accordance with a predetermined program. The caller inputs a calling code assigned in advance, and stored in the pager terminal apparatus carried by the callee of concern by way of push-buttons of a telephone and the like, in accordance with the given instructions. The calling code is temporarily stored in the pager base station. The caller further inputs a message which he intends to transmit to the callee. The pager base station 3 radio-transmits the given calling code and message in a predetermined communication format.
The input of the message from the caller is conducted as follows. In a control computer of the pager base station 3, there are stored standardized sentences to each of which a code is assigned for specifying the sentence, and a table for converting the given code to character strings of the standardized sentence. The caller selects a desired standard sentence referring to a table identical with this converting table, and assigns a code corresponding to the selected standard sentence by way of push-buttons and so on. The pager base station 3 converts the inputted code to a character string in accordance with the converting table. The base station 3 further radio-transmits the message including the character strings obtained by conversion along with the assigned calling code.
FIG. 2 shows one example of a communication format. Referring to FIG. 2, data to be transmitted includes a synchronous signal bit string "SYNC", a calling code "ADDRESS", and a transmission message "MESSAGE" including character strings from the caller.
In such a radio pager system, a short calling code inputted is converted to a message at the base station, so that it is possible to cut down the amount of key manipulation by the caller. However, a pager radiowave transmitted from the base station includes data converted to character strings. Therefore, in spite of the fact that there is a need to limit the amount of data in order to keep the traffic in an appropriate range for smooth communication, the amount of data transmitted is large. As character strings of a message are transmitted, as they are, by a radiowave which anyone can pickup, it is impossible to keep communication secret. The table for converting the fixed form of sentence code to character strings of a message is stored within the computer of the pager base station. Therefore, a large amount of work is needed for making converting tables fully corresponding to the needs of a multiplicity of subscribers. Even the message frequently used between a caller and a callee needs to have the whole sentence inputted as stated above if it is not included in the converting table of the base station. While it seems possible that this problem may be solved using an individual converting table with respect to a respective subscriber, it is technically impossible to prepare an individual converting table for each subscriber in the conventional pager base station.
If each user inputs all the messages without using such a converting table, he can transmit any messages. In this case, however, not only the amount of operation by the caller is increased, but also secret communication is prevented.